Time domain modulator for spectrum shaping of pulsed waveforms

ABSTRACT

A method and apparatus for controlling the spectral content of signals entering a radio frequency receiver. The frequency resolution of the receiver is improved by reshaping an incoming pulse waveform to a preselected pulse waveform having spectral characteristics which are defined and desirable thus permitting optimization of receiver characteristics without substantial loss of sensitivity.

United States Patent Tauscher et al.

[54] TIME DOMAIN MODULATOR FOR SPECTRUM SHAPING OF PULSED WAVEFORMS 325/472, 473, 479, 484, 321, 323, 324,13, 42, 44, 65; 328/164, 165; 329/106, 203; 332/9, ll; 333/70 T l2 l9 5 v BANDPASS VIDEO Fl L'l'ER AMPLIFIER [451 Oct. 17,1972

[56] References Cited UNITED STATES PATENTS 3,086,080 4/1963 Raisbeck ..s25/13 3,311,836 3/1967 Di Toro ..328/l64 3,358,129 12/1967 Schultz ..329/l06 Primary Examiner-Albert J. Mayer Attorney-Louis Etlinger 5 7] ABSTRACT A method and apparatus for controlling the spectral content of signals entering a radio frequency receiver. The frequency resolution of the receiver is improved by reshaping an incoming pulse waveform to a preselected pulse waveform having spectral characteristics which are defined and desirable thus permitting optimization of receiver characteristics without substantial loss of sensitivity.

12 Claims, 4 Drawing Figures 20 TIME DOMAIN SHAPING FUNCTION GENERATOR '4 l 22 r TIME DOMAIN RECEIVER SHAPING MODULATOR :PKTENTEDBBH? I 12 3.699.449

SHEET 1 BF 3 I2 l9 20 K TIME DOMAIN BANDPASS VIDEO SHAPING FUNCTION FILTER AMPLIFIER GENERATOR I E '4 2 TIME DOMAIN SHAPING MODULATOR R cEIvER I INVENTORS AGENT PATENTEDUBI 17 m2 SHEEI 2 [1F 3 TO MODULATORX 9 w gl AGENT PATENTEDUBT 17 m2,

sum 3 or 3 FREQUENCY FIG. 3(0) FREQUENCY 3E mw Om FIG.3(b)

lN VENTORS KENNETH ETAUSCHER THEODORE E ROOME,JR. DA D scoow M M AGENT BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates most generally to the field of radio frequency electronics and more particularly to a new and improved method and apparatus for reshaping input signal spectra to preselected narrow bandwidths.

2. Description of the Prior Art Prior to the present invention the most common approach to the problem of resolving a given radio tion through the prior art approach were on he order of -40 db.

OBJECTS AND SUMMARY OF THE INVENTION From the foregoing it will be understood that among the'various objectives of the present invention are:

To provide a new and novel method and apparatus for reshaping spectral sidebands of complex signals.

To provide a method and apparatus of the abovedescribed character employing modulation of received signals in the time domain.

To provide a method and apparatus of the abovedescribed character wherein arbitrary pulse spectra are reshaped to a preselected spectrum.

To provide a method and apparatus for modulating a signal in the time domain to provide spectrum shaping of said signal in the frequency domain.

These and other objectives of the present invention are efficiently attained by providing means for shaping a received radio frequency pulse signal to a preselected pulse shape thereby providing a preselected spectral shape prior to its application to a receiver or other signal processing apparatus. The received signal waveform is modulated by a selected function in the time domain and thus reshaped, providing spectrum shaping in the frequency domain. In this manner the received signal is applied to the receiver in such a form as to provide improved frequency resolution with a minimum power loss.

The foregoing as well as other objectives, features and advantages of the present invention will become more apparent from the following detailed discussion taken in conjunction with the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic block diagram of the apparatus of the present invention.

FIG. 2 is a schematic circuit diagram of a function generator of utility in the practice of the present invention.

FIGS. 3(a) and (b) are graphic illustrations of a radio frequency signal before and after reshaping by the apparatus of FIGS. 1 and 2.

DESCRIPTION OF PREFERRED EMBODIMENT Turning now to FIG. 1 there is illustrated in schematic block form a time domain pulse shaping apparatus in accordance with the principles of the present invention. A source of input signals such as a conventional receiving antenna 10 is coupled in the normal fashion to a bandpassfilter 12. As stated hereinabove, in the prior art the output of bandpass filter 12 would be coupled directly to the input preamplifier of a conventional receiver 14. In the practice of the present invention, however, the output of the bandpass filter 12 is electromagnetically coupled via coupler 16 through a video detector 18 and video amplifier 19 as a timing signal to a time domain shaping function generator 20. The filter 12 output is also coupled directly to a time domain shaping modulator 22, the operation of which is controlled by the function generator 20.

Examples of time domain modulating functions which have been found by the applicants to be of practical utility in the practice of the present invention include but are not limited to triangular, cosine-squared, and cosine functions. The implementation of the shaping function is such that the transfer function of the function generator 20 when multiplied by the transfer function of the modulator 22 yields the shaping function which is ultimately desired. The choice of specific shaping function is dependent upon the amount of spectral sidelobe reduction which is required in a given application.

The output of the bandpass filter 12 is applied to modulator 22 which for example may be a pin diode solid state variable attenuator such as a model SG-S53N which is commercially available from I-Iyletronics Corp. of Littleton, Massachusetts. Tl-Ie time domain modulated output of modulator 22, shaped for maximum spectral sidelobe reduction, is then coupled to the conventional receiver 14 or other suitable signal processing means.

With reference now to FIG. 2 there is shown a detailed schematic diagram of a cosine function generator which serves to illustrate the operation of one embodiment of the present invention. The output signal from the video amplifier 19 of FIG. 1 is a rectangular waveform of any given pulse-width, and is applied to the input 30 of the function generator of FIG. 2. The input 30 is coupled via a termination resistance 32 to ground and to an RLC cosine waveform generating circuit 34 comprising resistor 36 and capacitor 38, inductor 40 and diode 42 which are coupled in parallel to ground. The RLC waveform generating circuit 34 is preferably, although not necessarily, keyed to the narrowest pulsewidth input signal which is anticipated. The input pulse shock-excites the RLC waveform generating circuit 34 and the cosine shaped output signal produced thereby is coupled. to the input capacitor 44 of transistor amplifier circuit 46 which is coupled to a source of negative bias voltage, --V,. The NPN-transistor 48 and resistances 50, 52, 54, 56, and 58 of amplifier 46 are coupled together in the conventional manner well known in the art and for the purposes of brevity will not be discussed in detail.

The amplified cosine shaped output of amplifier 46 is coupled to a diode shaping network 60 which operates to reshape the cosine shaped signal to a form which, when multiplied by the transfer function of the modulator 22 of FIG. 1, is the ultimately desired cosine modulating function; i.e., the diode shaping network has a transfer function which is the compliment of the modulator transfer function. In this embodiment the input of the diode shaping network 60 is coupled through resistor 62 and potentiometer 64 to electrical ground potential. A biasing voltage, V,, is coupled via biasing resistor 66. Also coupled to the input via resistor 68 is a semiconductor diode 70. The anode of diode 70 is coupled through a potentiometer 72 to biasing voltage, -V,, and is also coupled through a blocking capacitor 74 to ground. A second semiconductor diode 76 has its anode coupled via capacitor 78 and potentiometer 80 to the source of biasing voltage, -V,. The cathode of diode 74 is coupled through resistor 82 to the output of the diode shaping network 60. The potentiometers 64, 72 and 80 may be adjusted such that the desired waveform is present at the output of the time domain shaping modulator 22 of FIG. 1.

The output voltage from the diode shaping network is applied to a voltage-to-current converter 84 comprising a PNP-transistor 86 having the negative biasing voltage, V,, applied to its collector and the emitter lead coupled to the base of NPN transistor 88. The emitter of transistor is coupled via resistor 90 to a negative biasing voltage, V;,.

The output current from the voltage-to-current converter is applied to the shaping modulator 22 of FIG. 1 and the interconnection is grounded through modulator current source resistor 92.

The input of the shaping function generator of FIG. 2 is also coupled to the output thereof through a modulator stored-charge dissipation circuit 94, comprising capacitor 96, diode 98 and resistor 100 the circuit being coupled to ground via resistor 102 and diode 104. When the input pulse is applied to the modulator stored-charge dissipation circuit any initial stored charge on the shaping modulator 22 of FIG. 1 is removed and distortion of the modulator transfer function is precluded.

While FIG. 2 is illustrative of a cosine shaping function generator, as stated hereinabove there any many different shaping functions which may be of utility in the practice of the present invention. For example at frequencies 100 MHz away from the RF carrier, amplitude modulation of a 400 nanosecond rectangular input pulse by a 400 nanosecond cosine-squared pulse provides spectral sidelobe reduction of about 70 db from the input spectral level. In similar fashion a triangular modulating waveform pulse-width-matched to the input provides a 36 db reduction and a cosine modulating wave form provides a 38 db reduction. A cosine-squared modulating waveform having a rise and fall time equal to five percent of the baseline pulse width provides a 24 db reduction in the sidelobe level from that of the rectangular input pulse. The carrier energy loss due to reshaping of the input signal is 6 db for rectangle to cosine-squared modulation and insertion losses are generally about -2 db. Thus a total carrier loss of only about 8 db is involved. This is also indicative of the magnitude of integrated energy loss in a given bandwidth.

FIG. 3(a) illustrates the spectrum of a pulsed signal including its many spectral sidelobes which in the prior art was applied directly to a receiver. FIG. 3(b) is illustrative of the relative degree of spectral sidelobe reduction achieved through the use of the apparatus of the present invention to reshape the input signal prior to its application to a receiver or other signal processing electronics. It will thus be apparent that the apparatus of the present invention provides a means for increasing the frequency resolution of an RF receiver by modulating input pulse waveforms to provide a preselected standard pulse waveform input to the receiver. The spectral characteristics of the standard pulse waveform are well defined and permit the optimization of the receiver parameters.

What has been described herein is what at present is considered to be the preferred embodiment of the invention. Many variations and modifications in the above-described structure will occur to those skilled in the art without departing from the scope of the invention and it is thus intended that all matter contained in the foregoing description or shown in the appended drawings shall be taken as illustrative and not in a limiting sense.

I-Iaving described what is new and novel and desired to secure by Letters Patent, what is claimed is:

1. Apparatus for shaping a pulsed waveform comprising a source of pulsed electromagnetic signals having a selected carrier frequency and a multiplicity of spectral sidelobes,

means coupled to said source for generating a modulation function comprising a pulse waveform of predetermined duration and shape in response to an electromagnetic signal pulse from said source,

a linear modulator coupled to said source of pulsed electromagnetic signals and to said modulation function generating means whereby said electromagnetic signal pulse is modulated in time by said modulation function to change the pulse envelope of said electromagnetic signal pulse,

said modulator being operative to reshape said electromagnetic signal pulse to a preselected spectrum and to reduce said spectral sidelobes to a selected level very much less than the level of said pulsed electromagnetic signal carrier frequency by forming the product of said signal pulse and modulation function, and

a frequency sensitive utilization means coupled to said modulator.

2. Apparatus as recited in claim 1 wherein said source of pulsed electromagnetic signals is a radio frequency receiving antenna, and

said modulation function generating means is electromagnetically coupled to said antenna output.

3. Apparatus as recited in claim 1 wherein said modulation function generating means includes a waveform generating circuit which generates a pulsed output signal of a selected waveform in response to a pulsed input signal, and a signal shaping network coupled to said waveform generating circuit and having a transfer function which is the substantial compliment of the transfer function of said modulator.

4. Apparatus as recited in claim 3 wherein said spectral sidelobes are reduced by at least 20 db from said carrier frequency sidelobes with a loss of less than 7 db at said carrier frequency. 9. Apparatus as recited in claim 1 wherein said pulse 5 waveform has a cosine-squared configuration.

10. Apparatus as recited in claim 1 wherein said pulse waveform has a triangular configuration.

11. Apparatus as recited in claim 1 wherein said 10 pulse waveform has a cosine configuration.

12. Apparatus as recited in claim 1 wherein said pulse waveform has a cosine squared-rectangularcosine squared configuration. 

1. Apparatus for shaping a pulsed waveform comprising a source of pulsed electromagnetic signals having a selected carrier frequency and a multiplicity of spectral sidelobes, means coupled to said source for generating a modulation function comprising a pulse waveform of predetermined duration and shape in response to an electromagnetic signal pulse from said source, a linear modulator coupled to said source of pulsed electromagnetic signals and to said modulation function generating means whereby said electromagnetic signal pulse is modulated in time by said modulation function to change the pulse envelope of said electromagnetic signal pulse, said modulator being operative to reshape said electromagnetic signal pulse to a preselected spectrum and to reduce said spectral sidelobes to a selected level very much less than the level of said pulsed electromagnetic signal carrier frequency by forming the product of said signal pulse and modulation function, and a frequency sensitive utilization means coupled to said modulator.
 2. Apparatus as recited in claim 1 wherein said source of pulsed electromagnetic signals is a radio frequency receiving antenna, and said modulation function generating means is electromagnetically coupled to said antenna output.
 3. Apparatus as recited in claim 1 wherein said modulation function generating means includes a waveform generating circuit which generates a pulsed output signal of a selected waveform in response to a pulsed input signal, and a signal shaping network coupled to said waveform generating circuit and having a transfer function which is the substantial compliment of the transfer function of said modulator.
 4. Apparatus as recited in claim 3 wherein the output signal of said waveform generating circuit is substantially pulse-width-matched to said pulsed input signal.
 5. Apparatus as recited in claim 3 further including a stored-charge dissipation circuit coupled between the input of said waveform generating circuit and the output of said signal shaping network.
 6. Apparatus as recited in claim 3 further including a voltage-to-current converter coupled to the output of said shaping network.
 7. Apparatus as recited in claim 1 wherein said modulator is a pin diode solid state variable attenuator.
 8. Apparatus as recited in claim 1 wherein said spectral sidelobes are reduced by at least 20 db from said carrier frequency sidelobes with a loss of less than 7 db at said carrier frequency.
 9. Apparatus as recited in claim 1 wherein said pulse waveform has a cosine-squared configuration.
 10. Apparatus as recited in claim 1 wherein said pulse waveform has a triangular configuration.
 11. Apparatus as recited in claim 1 wherein said pulse waveform has a cosine configuration.
 12. Apparatus as recited in claim 1 wherein said pulse waveform has a cosine squared-rectangular-cosine squared configuration. 